Implement scoresheet page with scoring logic and confirmation dialog

- Add src/game/scoring.ts with computeRunningTotals and isGameOver
- Build ScoreSheet grid: player columns, per-round scores, running totals after round 2+
- Small red "New Game" button in header top-right with full-screen confirmation overlay
- Extend PageFrame with headerAction and overlay props
- Move press animation/shadow from .pressable class to global button selector

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -43,6 +43,6 @@ export default function App() {
 
   if (page === 'title') return <TitleScreen onNewGame={handleNewGame} />
   if (page === 'setup') return <PlayerSetup onStart={handleStart} />
-  if (page === 'scoresheet') return <ScoreSheet onScoreRound={() => setPage('enterscores')} onNewGame={handleNewGame} />
+  if (page === 'scoresheet') return <ScoreSheet game={game!} onScoreRound={() => setPage('enterscores')} onNewGame={handleNewGame} />
   if (page === 'enterscores') return <EnterScores onSubmit={handleSubmitScores} />
 }